A very general active implantable system might look as depicted in FIG. 1. It consists of two implanted subsystems, a supply system 101, and a single chip processing system 102. The supply system 101 may contain input contacts 103 and 104, and output contacts 107 and 108. The input contacts are typically connected to each other via an inductive coil 105 (having an associated inductance L2). Implanted coil 105 might be inductively coupled to another external coil 106 (having its own inductance L1), which is positioned outside the body. Both coils form a weakly coupled transformer with external coil 106 as the primary winding and implanted coil 105 as the secondary winding. This allows a transfer of electrical energy via the intact skin surface (transcutaneous power transfer). The supply system 101 converts a radio-frequency (rf) signal u2(t) to an appropriate internal signal up(t) at the power supply output contacts 107 and 108. The power supply output contacts 107 and 108 are connected to the processing system input contacts 109 and 110 by isolated wires 111 and 112. Signal up(t) may supply the processing system 102 with both energy and information. Neglecting the electrical impedance of wires 111 and 112, signal up(t) also appears between processing system input contacts 109 and 110.
The processing system 102 typically performs particular measurement and/or active stimulation tasks, e.g., measurement of bio-electrical signals, sensing of chemical substances, and/or applying electrical signals to the surrounding tissue. Signals are sensed and/or applied by means of a set of electrodes 113. One special property of the processing system 102 is that, due to very restrictive spatial requirements, the whole functionality may be integrated on a single electronic chip. In contrast to the supply system 101 where electronic circuits might be protected against body fluids by means of a hermetically sealed package, the processing system 102 is typically protected only by various thin passivation layers (e.g., oxides). In addition, there may be no room for additional electrical components such as external diodes or discrete capacitors.
One specific example of a system as set forth in FIG. 1 may be a retinal prosthesis such as is described in Margalit E, Maia M, Weiland J D, Greenberg R J, Fujii G Y, Torres G, Piyathaisere D V, O'Hearn T M, Liu W, Lazzi G, Dagnelie G, Scribner D A, de Juan E, and Humayun M S, Retinal Prosthesis For The Blind, Survey of Ophthalmology, Vol. 47, No. 4, July-August 2002 (incorporated herein by reference). In that system, the processing system chip is located either on the surface of the inner retina (epiretinal approach) or in the subretinal space (subretinal approach). Typically, the size of the processing chip is some square millimeters, and the thickness is some tens of microns. For protection, the processing chip is covered by a several layers of light-transparent materials. The processing chip may include an array of subunits where each subunit includes a photodiode, an analog amplifier and a stimulating electrode. These subunits may be designed to convert the light energy (photons) from images into electrical impulses to stimulate the remaining functional cells of the retina.
Unfortunately, early hopes that such an implanted data processing chip could be powered solely by incident light without the use of external supply were not realized. Thus, the retinal processing chip has to be connected to a supply system providing power and control signals. For example, the supply system could be implanted in the area behind the ear, similar to a cochlear implant as described, for example, in Waltzman S B and Cohen N L, Cochlear Implants, ISBN 0-86577-882-5, Thieme New York, 2000 (incorporated herein by reference). Such a supply system could contain rechargeable batteries which could be recharged (if required) using a transcutaneous inductive link as described, for example, in Zierhofer C M and Hochmair E S, High-Efficiency Coupling-Insensitive Power And Data Transmission Via An Inductive Link, IEEE-Trans. Biomed. Eng. BME-37, pp. 716-723, July 1990 (incorporated herein by reference). Thus, a system configuration as shown in FIG. 1 is obtained.